Older adults show higher rates of right-handedness when compared to young adults. This is a well-established fact supported by many research papers (including some of my own) published since the 1980’s. What is less certain is the explanation for this age difference. A controversial theory, quickly found to be false, argued that left-handers have a shorter lifespan than right-handers so they are absent from samples of older adults. One can ignore this discredited theory, but two others deserve more attention. The first states that the environment of objects is arranged to favor the right-handed majority. As the years pass, individuals begin to use their right hands more frequently as an adaptation to this right-handed world. As a result, older adults show more consistent right-hand use across various activities when compared to younger age groups. The second theory claims that the left hemisphere of the brain, controlling the movements of the right hand, ages more slowly than the right hemisphere. As one gets older, the left hemisphere/right-hand control mechanism becomes more reliable and movements formerly done with the left hand or with both hands shift to the right side. This gradual shift results in more frequent right-hand use among older adults.
A 2019 study conducted in Brazil addressed the issue of age differences.¹ The authors compared the measured handedness of young adults (average age 31) to older adults (average age 70). They also assessed other types of lateral (side) preferences, namely, footedness, eyedness, and earedness. Each of these lateral preferences has a signature behavior. Handwriting is the signature behavior for identifying handedness. The signature behavior for footedness is the foot used to kick a ball, for eyedness, it is the eye that sights through a telescope or other one-eyed device, and for earedness, it is the ear where one places a phone. This study was unique because the authors measured a form of lateral preference called trunk laterality. I never heard of trunk laterality before reading this paper so I found this aspect of the research particularly interesting. Their measures of trunk laterality included the preferred side for sleeping and the preferred side used to push a heavy object.
This research replicated previous findings of higher rates of consistent right-handedness (79%) among older adults when compared to younger adults (60%). The other measures of lateral preference (except eyedness) also showed higher percentages of consistent right-sidedness among older adults. The novel measurement of trunk laterality revealed an age difference with 17% of older adults reporting a preference for the right side as compared to 8% of the young adult group.
The authors cited environmental influences as an explanation for their results. They proposed that sidedness becomes more asymmetric with age. Individuals develop an enhanced preference for the side of the body, the right side, that is used the most. Since the age difference was most pronounced for hand and foot preference, the authors suggested an individual might transfer tasks similar in form to the right limb over time. This results in a consistent preference for the right side.
Eyedness is a unique lateral preference that did not display a significant right-sided age difference. Unlike handedness and footedness, where movement control resides in the hemisphere opposite to the side of the limb, input from one eye reaches both hemispheres of the brain. Eyedness is not associated with a specific controlling hemisphere and, therefore, may not be subject to the same environmental influences as the limb lateral preferences.
This type of research assumes most people show right-sided preferences especially for the hand and foot. But what are the mechanisms behind the development of more consistent side preferences with age? A right-sided world supports the use of the right side and this asymmetry is strengthened over a lifetime of exposure as more behaviors shift to the right side. Lateral preferences are not fixed behaviors. They can be influenced by environmental arrangements and years of learning.
¹Marcori, A.J., dos Santos Grosso, N., Beggiato Porto, A, & Alves Okazaki, V.H. (2019). Beyond handedness: assessing younger adults and older people lateral preference in six laterality dimensions. Laterality: Asymmetries of Body, Brain and Cognition, 24, 163-175. https://doi.org/10.1080/1357650X.2018.1495725